U.S. Pat. No. 5,289,922 to Inculet et al. discloses electrostatic separation of mixed plastic waste. Inculet et al. discloses an apparatus for tribocharging particles of insulating materials. The apparatus includes a vessel for containing the particles, and the vessel is of such material that the work function of the material of the vessel is different from that of the materials of at least some of the particles in the vessel.
The vessel is rotatable, and the apparatus includes a means for rotating the vessel about an axis of rotation. The orientation of the axis of rotation is such that, in operation of the apparatus, the particles tumble against each other and against the walls of the vessel, thereby becoming tribocharged.
The mixture of the plastic materials to be separated is fed into the vessel at an entry point thereof, and emerges from the tube at an exit point thereof. The apparatus further includes means for gradually and progressively moving the material through the vessel from the entry point to the exit point; and the arrangement of the apparatus is such that the material at any point in the progression there between is kept separated from the material at other points in the progression.
The progression of the particles through the vessel is such that the particles do not mix generally, i.e. with particles at different levels of charge, but only with particles at the same level of charge. Therefore, the charge on the particles is enabled to increase progressively, without being dissipated, as the particles travel through the vessel.
The vessel is preferably in the form of a tube, which is open at each end, with the tribocharged particles entering at one end and emerging from the other end of the tube. The tube is mounted for rotation, and the axis of rotation of the tube is inclined at a slight angle to the horizontal, downwardly from the inlet to the outlet. The speed of rotation of the tube, and the said slight angle, are preferably adjustable.
The tube may include at least one rib of the same material as the tube, the rib extending radially inwardly from the inside surface of the wall of the tube. The rib is disposed along the axial length of the tube, so that as the tube rotates, the rib serves to tumble the mixture of particles residing in the tube.
The separation apparatus may further include a channel positioned in line with the end of the tube, the channel having at least one opening (e.g. an elongated slot, preferably tapered, or a plurality of small holes) along the bottom thereof for particles to drop therethrough as a curtain of particles, high voltage electrodes being arranged vertically on either side of the channel and extending there along and downwardly to separate the particles.
The aim of this construction is to provide a means of triboelectrification, which will operate highly effectively and with less capital cost and expenditure of energy than in traditional fluidized bed apparatus. Although the apparatus disclosed by Inculet et al. does achieve that aim, there are some drawbacks to the apparatus disclosed therein that this invention purposes to overcome.
Since in the electrostatic separation process, separation is based on charge to mass ratio and this ratio increases linearly when particle size decreases, uniform particle size is an important parameter in determining the purity and yield in a separation of mixed plastics. Further, fines which charge very strongly, can undesirably collect on the electrodes decreasing the electric field strength as the deposit builds up and thereby degrade the separation efficiency with time.
At one stage of the separation process, as disclosed by Inculet et al., is a channel positioned in line with the end of the tube, the channel having a plurality of openings or a tapered slot along the bottom thereof for particles to drop therethrough as a “curtain”. High voltage electrodes are arranged vertically in the form of a tower on either side of the channel and extend there along and downwardly to separate the particles. The drawback in that system is that there is no instantaneous or continuous feedback to the operator of the quantity of the material distribution being separated.
To obtain a good separation and high particle throughput rates, a continuous stream of particles in a line parallel to the electrodes is required from the stage described above since the separation field acts only in a direction perpendicular to the electrodes. Also, the initial velocity of the particles entering the tower should be as uniform and as small as possible. However, both desirable qualities are not sufficiently achieved in the prior art.